How to get early feedback prior to the official submission to Apple

Yes, submitting your app via TestFlight for public beta testing serves that purpose. The app will undergo a review, albeit a smaller review than the ordinary App Store review. You do not need to actually release your app for beta testing to any third party (except Apple) - you can choose to get it approved and then do nothing.

Similarly you can submit your app for the App Store to get the full review done. There's no need for you to actually make the app public on the App Store right away - you can keep it out of the App Store for as long as you like - and publish only when you're ready. It is also possible to update the app before publishing - although the update will then have to undergo review as well.

You do not have to worry about your official submission being "tarnished". If Apple rejects the app, you have plenty of options for changing the app and resubmitting for a new review.

In the interest of saving time, it is however a good idea to make sure that the app follows the new guidelines before submitting them it to review.